Conventional gas analyzers have been used for many purposes including measuring gas concentration of gases in a gas stream. Such measurements often are used to measure emissions from furnaces. That information can be used to determine the overall efficiency of the boiler as a heat producing device and to show compliance with environmental regulations.
An important category of gas sampling relates to the compliance monitoring requirements enforced by the United States Environmental Protection Agency (EPA). Many sources of air pollution, such as fossil fuel power plants, incinerators, metal smelters, and cement kilns are required to monitor levels of certain gaseous species that are released into the atmosphere. These species include sulfur dioxide, nitrogen oxide, carbon monoxide, carbon dioxide and oxygen.
The gas streams to be monitored typically have certain intrinsic characteristics which complicate testing. For example, they generally contain 6% to 20% by volume of evaporated moisture. It may be necessary to remove that moisture or know the amount of moisture present before further analysis can be done.
The conventional practice in emissions monitoring has been to insert a probe into the stack to draw off a gas sample. The sample is then directed to a gas analyzer at a remote location. This is necessary because conventional monitors would be adversely affected by the environmental conditions at the stack which could include wide temperature ranges from well below freezing to over 100.degree. F., high winds and particulates in the air. In my U.S. Pat. No. 5,297,432 I disclose a vacuum dilution extraction gas sampling method which relies upon vacuum transport of the gas sample to a remote analyzer. In these monitoring environments the amount of gas to be measured typically is in quantities of a few parts per million. Hence, the analyzer must be quite sensitive.
In U.S. Pat. No. 4,724,700, Jasma discloses a differential flow gas analyzer. The gas sample is filtered and directed through a first orifice. Then the sample is passed through a condenser and a drying column to remove moisture. The dry gas sample is then directed through a second orifice. Jasma measures the flow rate of the gas sample before the second condensing step and after that step. Then, he uses the flow rates to determine the concentration of the moisture removed from the sample.
Hirsch et al. in U.S. Pat. No. 4,507,078 discloses a device for determining the concentration of condensable vapor in a flowing gas stream. The gas sample is passed through a condenser and then through a gas flow meter at the discharge of the gas condenser. The gas samples are drawn in such a manner that the flow rate into the condenser is known. The amount of condensable vapor concentration is determined by comparing the flow of the dry gas sample with the known volumetric flow to the condenser. One problem with the methods disclosed by Hirsch and Jasma is that they are not suitable for measuring low concentrations of the gas of interest in the sample. This is true because the method relies upon finding a difference between the flow rates at two distinct points. Since the sensitivity of flow meters available in the marketplace is limited, relying on the difference between two such readings makes it very difficult to determine the gas concentration if 0.01% or less of the sample is the gas of interest. Hence, these systems are not suitable for emissions monitoring in several industries.
There has long been a need for a simple yet effective gas concentration monitor that employs well-established technology in a cost effective reliable manner. This monitor should be capable of measuring a wide range of concentration in the order of 50% to less than one hundred parts per million.